1. Field of the Invention
The present invention is related to methods for degrading certain aromatic pollutants which generally resist degradation in the environment. More particularly, the present invention is related to methods for using white rot fungi, or an enzyme system produced by the fungi, for degrading coal tar, coal tar distillation fractions and constituents, and aromatic organic compounds (particularly polycyclic aromatic compounds) of the type generally found in coal tar or its distillation fractions.
2. Background of the Invention
Disposal of environmental pollutants is of increasing concern in this country and worldwide. Proper disposal is necessary in order to avoid contaminating the environment and poisoning organisms living in a particular environment. In the past, relatively casual disposal procedures have been employed for almost all pollutants. For example, pollutants were often simply dumped into a river, ocean, or other body of water. When water was not available, pollutants were often buried or spread across an area of land. While disposal procedures have improved dramatically, generally in response to increased environmental regulation, the effects of failing to properly dispose of pollutants in the past still persists.
Some pollutants are not easily degraded in the natural environment. As a result, these persistent pollutants may be found at disposal sites years, and even decades, after disposal. These pollutants continue to threaten the environment and individuals living in the environment until the pollutants are finally removed or degraded by extraordinary means.
Some of the most persistent and toxic environmental pollutants comprise cyclic organic molecules; large quantities of which have been disposed of improperly. Chemical manufacturing plants and industrial sites using such chemicals are some of the major sources of these chemicals.
One major source of cyclic organic molecules is coal tar and related compounds. Cyclic organic compounds are often found as constituents of coal tar. The present invention relates to methods for degrading coal tar, coal tar distillation fractions, and cyclic organic molecules of the type generally found in coal tar, whether derived from coal tar or another independent source.
In the past, coal tar and related materials have been produced widely and in large quantities. Unfortunately, disposal methods have been extremely casual and inadequate.
One major source of pollution by coal tar and coal tar related organic compounds was the gasification of coal and oil prior to the widespread use of natural gas. Coal and oil gasification processes produced coal tar and coal tar related compounds in great abundance. As a result, almost every utility operating during the period of time in which coal and oil was commonly gasified has an associated disposal site. It is currently estimated that there are over 2,000 major coal tar disposal sites in the United States alone.
Coal tar and related aromatic organic compounds are also widely used in industry, including the utility and railroad industries. One use of coal tar and coal tar distillation fractions is in treating wood for use as railroad ties and utility poles. This treatment prevents degradation of the wood while it is exposed to the environment. The very features of coal tar and coal tar distillation fractions which make them usable as wood preservatives, however, also make them difficult to dispose of in the environment.
Many of the component molecules of coal tar are not easily degraded by microorganisms found in nature. Coal tar consists of numerous different components. One of the commonly used distillation fractions of coal tar is creosote. Another distillation fraction of creosote is anthracene oil. The present invention is directed to a process for degradation of coal tar distillation fractions of this type, as well as coal tar itself.
Coal tar also contains several different aromatic organic molecules. These range from single ring compounds such as p-cresol, to multiple ring compounds such as benzo(a)pyrene. Intermediate cyclic organic molecules are also commonly found in coal tar. For example, naphthalene and 2-methylnaphthalene are constituents of coal tar. Degradation of all of the above classes of organic molecules, whether derived from coal tar or from another source, fall within the scope of the present invention.
As mentioned above, aromatic hydrocarbons are difficult to degrade in nature. Most microorganisms are unable to significantly degrade many of these molecules, particularly the more complex multiple ring aromatics. Polycyclic aromatics are, therefore, persistent organic pollutants for which those skilled in the art have long sought to develop adequate and safe methods of disposal. Once these compounds have been disposed of in the environment, they break down only slowly, if at all. Thus, large quantities of these compounds disposed of forty (40) to fifty (50) years ago may still yet be found today at a disposal site.
A serious related environmental problem is that some of these compounds, particularly lighter aromatic hydrocarbons, are at least somewhat water soluble. For example, p-cresol is known to be relatively water soluble. Double ring compounds such as 2-methylnaphthalene are also found to be at least somewhat water soluble. Accordingly, these compounds present a serious threat to ground water. Water flowing through a disposal site will leach some of the soluble hydrocarbons from out of the site. These hydrocarbons can then be introduced into the general ground water.
As mentioned above, most microorganisms are not able to cause complete biodegradation of polycyclic aromatic compounds. A few species of bacteria and fungi are able to cause degradation of relatively simple aromatics. However, polycyclic aromatic compounds, such as benzo(a)pyrene, are generally very resistant to biodegradation.
Although some microorganisms have been found which are able to degrade certain polycyclic aromatic compounds, their usefulness in microbial waste systems is often limited. Degradation using these species is not generally extensive and does not usually proceed to degrade the polycyclic aromatic compounds to carbon dioxide and water as end products of metabolism. Unfortunately, some metabolites of degradation may be more toxic than the parent compound and these intermediates may also be environmentally persistent. Thus, incomplete degradation can create a situation which is worse than the original problem.
Microorganisms conventionally used in waste treatment systems are generally selective in the compounds they act upon and are not able to degrade a wide variety of organo-pollutants. Thus, mixtures of pollutants are not totally biodegradable using the processes of the prior art. Attempts have been made to degrade such mixtures with mixtures of microorganisms. This type of system has, however, met with limited success and is extremely complex in its application.
It would, therefore, be a major advancement in the art to provide a process for the degradation of coal tar, distillation fractions and constituents of coal tar, and compounds often associated with coal tar including cyclic and polycyclic aromatic compounds. It would be a related advancement in the art if such a process could be used directly in the environment such that coal tar disposal sites could be treated in situ. It would be a further advancement in the art to provide simple, yet economical methods for the degradation of these compounds.
It would also be an advancement in the art to provide methods which were thoroughly effective and which left no toxic intermediates in the degradation process. Similarly, it would be an advancement in the art to provide a process for thoroughly degrading coal tar, its constituents, and related pollutants using a single microorganism.
Such methods are disclosed and claimed herein.